It will effect his application, it depends on many factors:
1. Is it on his medical record?
2. Did he have medication?
3. What were the dates?
I was defered last month untill Aug because of depression, i had an entry of this on my medical records in March 07, but i had 6 months medication untill aug 07, which is why i have to wait. They want to see a year without any problems from the last date on the records, whether that is medication or just visiting the doctor. The good news like me as long as everything else is ok (medical) wise if you get defered you dont have to do any forms again you just can't go on selection untill that date.
Due to recent experiance get a line from his Civ Doc as I have just lost a lad due to having depression 4 years ago, He was/is mad keen on joining up but due to the depression tag he is being turfed out. The cause of his depression was an ex bird so she has shafted him twice now.